Question 1

Horton sphere storage for liquid ammonia: identify the incorrect statement about dimensions, support, working pressure, and service conditions.

Accepted Answer

Introduction / Context: Horton spheres are spherical storage vessels widely used for liquefied gases (e.g., ammonia, LPG). Their geometry gives favorable stress distribution and minimum surface area per unit volume. This question checks practical knowledge about their dimensions, supports, and pressure ranges. Given Data / Assumptions: Spherical storage for ammonia and similar volatile liquids. Conventional carbon steel construction with standard supports. Moderate pressures for refrigerated or ambient storage. Concept / Approach: Spheres use columns/legs for support and commonly range from a few meters to a few tens of meters in diameter. Because of uniform membrane stress, required thickness is lower than for a cylindrical shell at the same pressure. Operating pressures for such storage are typically moderate; extremely high pressures (hundreds of kg/cm2) are not used for bulk storage spheres, especially at sub-zero conditions where low-pressure refrigeration is favored. Step-by-Step Solution: Check geometry benefits: sphere has minimum surface area per volume and uniform stress.Confirm support details: multiple columns are typical for large spheres.Evaluate pressure claim: “very high pressure up to 200 kg/cm2” for sub-zero liquids is unrealistic for Horton spheres; wrong statement identified. Verification / Alternative check: Industry practices for ammonia storage employ refrigerated low-pressure tanks or pressurized spheres at modest pressures—far below hundreds of kg/cm2. Why Other Options Are Wrong: (a) Typical size and supports are correctly described. (b) Spherical shells indeed need less thickness and insulation than comparable cylinders. (c) Moderate-pressure service is a standard application for Horton spheres. Common Pitfalls: Confusing transport cylinders (high-pressure) with large fixed storage spheres (moderate-pressure). Also, assuming low temperature always implies high pressure. Final Answer: Used for storing liquids at sub-zero temperature at very high pressure up to 200 kg/cm2.

Question 2

Piping practice: Normal screwed (threaded) fittings are typically used up to what nominal pipe size (inches)?

Accepted Answer

Introduction / Context: Choosing a jointing method (threaded, flanged, welded) depends on pipe size, pressure, and service. Threaded (screwed) fittings are convenient for small diameters but become impractical and unsafe as size and pressure increase due to leakage and make-up torque limitations. Given Data / Assumptions: Industrial service with standard carbon steel piping. “Normal” threaded fittings—i.e., routine plant practice. Moderate pressures and temperatures. Concept / Approach: Threaded joints are common in small-bore piping because assembly is simpler and cheaper. As diameter grows, the risk of leakage, thread galling, and low mechanical rigidity rises. Many design handbooks recommend limiting routine use to small sizes; typical conservative limits are ≤ 1.5 inch in many plants (with some practices extending to 2 inch for benign service). Beyond that, flanged or welded connections are preferred. Step-by-Step Solution: Identify common practice: keep threaded joints to small NPS.Select the closest standard limit provided in choices (1.5 in.).Recognize that larger sizes increase leakage and assembly difficulty; therefore 3 in and above are unsuitable. Verification / Alternative check: Company standards and codes of construction commonly cap threaded joints at small nominal sizes for pressure-rated service. Why Other Options Are Wrong: 3, 5, 7 inches: too large for “normal” threaded practice; flanged or welded joints are preferred. Common Pitfalls: Applying small-bore rules to high-pressure/high-temperature or hazardous service—threaded joints may be prohibited regardless of size in such cases. Final Answer: 1.5

Question 3

Perforated plate with tubes through holes: If hole diameter is dn on pitch ph and each hole carries a tube of inner diameter dt, what is the ligament efficiency?

Accepted Answer

Introduction / Context: Ligament efficiency measures the fraction of solid material left between adjacent openings and is important in mechanical integrity of tube sheets and perforated plates. When tubes pass through holes, the relevant dimension that removes load-bearing material is the tube outside/inside diameter as applicable. Given Data / Assumptions: Hole diameter denoted dn, hole pitch ph. Each hole contains a tube of inner diameter dt (the opening through the plate is effectively dt for fluid passage and ligament calculation based on the free gap). Simple linear pitch in one direction considered. Concept / Approach: Ligament efficiency along one pitch is defined as the ratio of the net ligament width to the pitch: η = (ph - effective opening width) / ph. When a tube passes through the hole, the effective opening replacing solid material is the tube diameter used in the stress path—here specified as dt. Hence η = (ph - dt) / ph. Step-by-Step Solution: Start with η = net ligament / pitch.Net ligament = ph - effective circular opening size along the pitch direction.Use given “tube I.D. dt” as the effective opening → η = (ph - dt) / ph. Verification / Alternative check: In tube-sheet design, similar relations use tube outside diameter when structural metal is removed to accommodate the tube. Here, the problem states dt explicitly for the opening dimension, justifying the result. Why Other Options Are Wrong: (a) uses dn (bare hole) and ignores the tube; not aligned to the stated geometry. (b) introduces an incorrect mixed term dn . dt / 2. (d) normalizes by dn instead of pitch, which is not the standard definition. Common Pitfalls: Confusing the appropriate diameter (hole vs. tube). Always use the dimension that removes load-carrying ligament in the direction being evaluated. Final Answer: (ph - dt) / ph

Question 4

Sizing of sieve-tray columns: the column diameter is primarily fixed by which vapor-related criterion?

Accepted Answer

Introduction / Context: For trayed distillation/absorption columns, diameter selection is driven by hydraulics. Excess vapor velocity causes entrainment and flooding, while too low a velocity yields poor mass transfer efficiency and weeping. Balancing these constraints sets the operating window; the upper limit usually governs diameter. Given Data / Assumptions: Sieve (perforated) trays with typical weir and downcomer geometry. Continuous operation targeting normal turndown and peak loads. Concept / Approach: Column cross-sectional area A is chosen so that superficial vapor velocity U remains below a design fraction of the flooding velocity. Since vapor volumetric rate dictates U = V/A, ensuring U ≤ U_max (from flooding correlations) fixes the required area and therefore diameter. Hole size and plate spacing are secondary design choices affecting efficiency and pressure drop, but do not primarily set diameter. Step-by-Step Solution: Estimate vapor rates from material and energy balances.Use flooding correlations to compute allowable U_max.Choose diameter to keep U below the allowable fraction (e.g., 70–85% of flood). Verification / Alternative check: Tray rating methods and vendor software confirm diameter determined mainly by vapor hydraulic constraints; sensitivity checks with hole size/spacing show smaller impact on required diameter. Why Other Options Are Wrong: Lower limit of vapor velocity sets weeping limit and turndown, not initial diameter. Hole diameter and plate spacing influence efficiency/pressure drop but do not dominate diameter sizing. Common Pitfalls: Ignoring liquid handling (downcomer flooding) and assuming a single “vapor-only” criterion; both vapor and liquid hydraulics must be checked, but the upper vapor limit predominates for diameter. Final Answer: Upper limit of vapor velocity (to avoid flooding/entrainment)

Question 5

Storage tank design: Which roof type is most commonly provided on cylindrical storage tanks for general service?

Accepted Answer

Introduction / Context: Fixed-roof cylindrical tanks are widespread for liquids such as water, light hydrocarbons (when not requiring floating roofs), and chemicals. Roof geometry affects structural behavior, rainwater shedding, fabrication simplicity, and cost. Given Data / Assumptions: Atmospheric or low-pressure fixed-roof tanks for general liquid service. No special internal pressure demanding domed roofs. Concept / Approach: Conical roofs are simple to fabricate with plate segments, provide efficient drainage, and achieve adequate strength with rafters and columns where needed. Flat roofs are uncommon for outdoor service due to poor drainage. Domed (spherical segment) roofs are used on low-pressure tanks and larger diameters but entail more complex fabrication; “umbrella” is a descriptive variant but not the standard choice for general service. Step-by-Step Solution: List common roof types and their typical applications.Match to “most commonly used” for standard cylindrical tanks → conical roof.Exclude flat roofs due to drainage and structural downsides; domes are specialty or low-pressure applications. Verification / Alternative check: Tank standards and vendor catalogs show conical roofs as the default for many fixed-roof designs, with options for self-supporting or supported configurations. Why Other Options Are Wrong: Flat: drainage and ponding issues. Dome: often used for low-pressure/vapor-tight tanks but not the most common across general services. Umbrella: not the prevalent standard term/choice in typical specifications. Common Pitfalls: Assuming floating roofs (used for volatile products) fit the question; those are different from fixed roofs addressed here. Final Answer: Conical roof

Question 6

Mixing equipment design: For a paddle agitator, what is the typical impeller-to-tank diameter ratio used as a design rule of thumb?

Accepted Answer

Introduction / Context: Agitator geometry strongly influences flow patterns, power draw, and mixing time. A fundamental geometric parameter is the impeller diameter-to-tank diameter ratio (D/T). For paddle agitators (flat blades), standard proportions are used to ensure circulation and suspension without excessive power consumption. Given Data / Assumptions: Cylindrical tank with standard baffles or unbaffled as commonly used with paddles. Ambient liquid mixing without gas dispersion considerations. Concept / Approach: Paddle agitators produce primarily radial flow. To be effective across the tank cross-section, a relatively large impeller diameter is used. Typical D/T values for paddles are around 0.4–0.5, whereas high-efficiency axial-flow impellers (e.g., pitched blade turbines, hydrofoils) often use smaller D/T ~0.3–0.35 with higher speed. Choosing D/T ≈ 0.5 for paddles provides robust circulation in viscous or moderate-viscosity services. Step-by-Step Solution: Relate flow regime and impeller type: paddle → radial flow, larger D/T.Apply rule-of-thumb range; select representative value 0.5 from options.Confirm that too small (0.1–0.25) would under-mix; too large (0.8) is impractical and power-intensive. Verification / Alternative check: Vendor literature and handbooks list D/T ≈ 0.4–0.5 for paddles depending on duty and viscosity. Why Other Options Are Wrong: 0.1 and 0.25: impeller too small to sweep the cross-section effectively. 0.8: excessively large, causing high torque/power and mechanical issues. Common Pitfalls: Confusing paddle with axial-flow impellers that use smaller D/T but higher rotational speeds. Final Answer: 0.5

Question 7

Pressure vessel classification: A vessel is considered “thin-walled” when the ratio of diameter to wall thickness satisfies which condition?

Accepted Answer

Introduction / Context: Thin-wall versus thick-wall assumptions determine which stress equations apply for cylinders and spheres under pressure. The thin-wall approximation simplifies analysis by assuming uniform membrane stress and negligible through-thickness stress variation. Given Data / Assumptions: Prismatic cylindrical vessels under internal pressure. Elastic behavior within code-allowable stress. Concept / Approach: A common rule is that a shell can be treated as thin-walled when the diameter-to-thickness ratio D/t is sufficiently large (or equivalently, when t/D is small). A widely used threshold is D/t > 10 (some texts use > 20 for a more conservative thin-wall assumption). The question asks for the basic condition; hence “> 10” matches standard teaching definitions. Step-by-Step Solution: Define thin-wall condition: negligible radial stress gradient.Adopt typical criterion: D/t greater than about 10 implies thin-wall behavior.Select the option “> 10.” Verification / Alternative check: Comparisons of Lamé thick-cylinder equations with thin-wall formulas show small error once D/t exceeds ~10–20, supporting the rule of thumb. Why Other Options Are Wrong: < 10 indicates thicker walls where thin-wall formulas may be inaccurate. > 20 is more conservative but not the minimal defining threshold commonly taught. “30” as a stand-alone number lacks inequality context. Common Pitfalls: Using thin-wall stress formulas outside their validity; always check code requirements and joint efficiency. Final Answer: > 10

Question 8

Tray design for bubble-cap columns: Which variable primarily determines the number of bubble caps per tray?

Accepted Answer

Introduction / Context: Bubble-cap trays distribute vapor through caps and slots into liquid on each tray. Proper sizing ensures adequate contact area while avoiding excessive pressure drop and entrainment. The number of caps must match the vapor traffic that the tray must handle at design conditions. Given Data / Assumptions: Conventional bubble-cap geometry and slot design. Design at steady-state capacity with some turndown margin. Concept / Approach: Vapor load sets the total required open area for vapor passage. That total area is provided by the combined slots of all caps. Thus, for a given cap type (slot area per cap) and allowable slot velocity, the number of caps N must satisfy N * A_slot_per_cap * v_allowable ≥ total vapor volumetric flow. Tray diameter and liquid load influence layout and hydraulics, but the count of caps scales directly with vapor flow requirement. Step-by-Step Solution: Compute vapor volumetric rate at tray conditions.Fix allowable slot velocity from design correlations.Choose cap type (slot area per cap) and solve for the number of caps to provide total slot area. Verification / Alternative check: Vendor rating methods for bubble-cap trays perform the same balance between vapor flow, slot area, and slot velocity, yielding the number of caps. Why Other Options Are Wrong: Permissible slot velocity is a design parameter used in the calculation, not the driving plant load. Tray diameter constrains physical placement but does not set the required count by itself. Liquid load affects weir/load hydraulics but not the primary sizing of cap count. Common Pitfalls: Ignoring maldistribution and maintenance access; practical layouts may require rounding up the cap count and adjusting spacing. Final Answer: Vapor load

Question 9

Packed columns near limiting operation: At (or toward) the flooding region for a given packing and fluids, how does pressure drop per metre of packed height respond to increases in fluid rates or operating pressure?

Accepted Answer

Introduction / Context: In packed towers (absorption, stripping, distillation), hydraulic behavior dictates capacity limits. As gas and/or liquid rates rise, pressure drop increases. Approaching flooding, liquid holdup grows rapidly, channels fill, and pressure drop escalates sharply, signaling the operational limit. Given Data / Assumptions: Fixed packing and fluid pair. Operation pushed toward the flooding point (capacity limit). Steady-state behavior without foaming or fouling complications. Concept / Approach: Frictional drop in packed beds stems from gas flow through wetted voids and liquid holdup. Increasing gas rate boosts shear and entrainment; increasing liquid rate thickens films and holdup. Near flooding, the void fraction available to gas collapses and ∆P/m rises dramatically. Higher operating pressure (at fixed mass rates) can reduce gas volumetric flow, but “toward flooding,” the governing trend with rising load is a steep ∆P rise. Step-by-Step Solution: Recognize capacity limit: flooding marked by sudden ∆P/m increase.Increase fluid rates → higher holdup and gas–liquid interaction → ∆P/m rises.Therefore, near the flooding region, pressure drop per metre increases with load. Verification / Alternative check: Design charts (e.g., generalized pressure drop correlations) show steepening pressure drop curves as approach to flooding is made, confirming the trend. Why Other Options Are Wrong: Decrease or no change contradicts observed hydraulics near flooding. Dependence on gas solubility is secondary; hydraulic limit is dominated by two-phase flow, not dissolution. Common Pitfalls: Confusing “loading” with “flooding.” Loading is the onset of liquid accumulation; flooding is the capacity limit with runaway ∆P. Operators should maintain adequate margin below flooding. Final Answer: Increases

Question 10

Shell-and-tube heat exchanger design: For a shell inside diameter D, what is the usual range of baffle spacing selected by designers (include the standard minimum spacing in millimetres/inches and the common upper bound as a fraction of D)?

Accepted Answer

Introduction / Context: Proper selection of baffle spacing in a shell-and-tube heat exchanger controls shell-side heat transfer, pressure drop, vibration risk, and flow-induced tube erosion. Designers typically tie baffle spacing to the shell inside diameter (D) and enforce a practical minimum distance to accommodate construction and cleaning. Given Data / Assumptions: Baffles are single- or double-segmental on the shell side. Inside shell diameter is D. Common industry rules-of-thumb are used for initial sizing before detailed rating. A practical minimum spacing (often near 2 inches ≈ 50 mm) is observed for fabrication and bundle robustness. Concept / Approach: Baffle spacing governs crossflow velocity and hence the shell-side heat transfer coefficient and pressure drop. Too small a spacing increases pressure drop and vibration risk; too large a spacing reduces crossflow, lowering heat transfer and allowing flow maldistribution. A widely used preliminary range is from about D/5 up to about D, with a hard floor around 2 inches. Step-by-Step Solution: Relate spacing to diameter: lower bound ≈ D/5 (about 0.2D) for sufficient crossflow.Upper bound ≈ D to limit bypassing and maintain reasonable performance.Apply construction limit: absolute minimum commonly ≈ 2″ to allow tube support and cleaning access. Verification / Alternative check: Preliminary TEMA-style guidance and vendor practices consistently recommend a first-cut spacing between 0.2D and 1.0D, subject to vibration checks and allowable pressure drop limits. Why Other Options Are Wrong: D/2 or minimum 2″ to 5D: the 5D upper bound is excessively large and not standard. D/5 or minimum 2″ to 5D: again, the 5D limit is not typical. None of these: incorrect because D/5 or minimum 2″ to D is the accepted range. Common Pitfalls: Using very tight spacing that creates high pressure drop; ignoring tube vibration checks at high crossflow velocity; failing to consider cleaning requirements for fouling services. Final Answer: D/5 or minimum 2″ to D

Question 11

Distillation tray detailing: What is the purpose and recommended placement/size of weep holes provided in tray decks, and which combined statement best captures industry practice?

Accepted Answer

Introduction / Context: Weep holes are small openings in distillation tray decks that ensure reliable drainage at shutdown and help maintain tray cleanliness. Their sizing and placement affect start-up behavior, fouling tolerance, and unintended weeping during operation. Given Data / Assumptions: Perforated trays with downcomers and an overflow weir are considered. Unit may process fouling or clean services. Weep holes exist in addition to active perforations/valves/caps. Concept / Approach: In shutdown or low-vapor scenarios, liquid needs an escape path to prevent pooling and corrosion. Placing weep holes near the weir aligns any residual drainage path with normal liquid flow, preventing stagnant zones. Hole diameter must balance fouling tolerance (too small plugs easily) with operational integrity (too large causes excessive leakage at turndown). Step-by-Step Solution: Purpose: allow drainage when vapor traffic is absent; reduce stagnant liquid.Placement: near the overflow weir so draining follows the normal liquid route.Sizing: typically about 1/4″ to 5/8″ diameter to mitigate plugging while avoiding undue operating weep. Verification / Alternative check: Tray design manuals and vendor guidelines specify limited weep-hole areas and strategic placement to avoid bypassing active bubbling zones. Why Other Options Are Wrong: Only drainage: incomplete; placement and size matter. Placement-only: omits the function and sizing criterion. Size-only: omits purpose and placement rationale. Common Pitfalls: Oversizing holes leading to weeping at low vapor rates; randomly placing holes and creating maldistribution or dead zones; neglecting fouling characteristics when selecting size. Final Answer: All of (a), (b) and (c)

Question 12

Fluid mechanics loss coefficients: For a tee used as an elbow (flow enters the branch), how does its loss coefficient compare to that of a tee used as an elbow (flow enters the run)?

Accepted Answer

Introduction / Context: Minor loss coefficients (K) in fittings quantify pressure drops due to flow separation and secondary motion. Tees configured as elbows are common in process piping, and the orientation of the entering leg (branch versus run) strongly influences K. Given Data / Assumptions: Incompressible single-phase flow. Standard commercial tee fitting. K values referenced at turbulent Reynolds numbers typical of process services. Concept / Approach: When flow enters the branch and turns into the run, the geometry forces a sharper turn and stronger separation than the reverse case. The consequent energy dissipation is higher, so the loss coefficient is larger for entering-branch service than for entering-run service. Step-by-Step Solution: Define cases: Case 1 — tee as elbow, entering branch. Case 2 — tee as elbow, entering run.Compare turning severity: entering branch induces more abrupt change in momentum.Result: K_branch-entry > K_run-entry, hence greater pressure drop in Case 1. Verification / Alternative check: Crane TP-410 and similar handbooks list tabulated K values showing higher coefficients for entering the branch versus entering the run, all else equal. Why Other Options Are Wrong: Less than: contradicts standard data. Approximately 90: meaningless here; K is dimensionless and not fixed to 90. Both (b) and (c): includes an incorrect assertion. Common Pitfalls: Confusing branch and run nomenclature; reusing K values from straight-through tees for turned-flow cases; ignoring velocity head reference area consistency. Final Answer: More than that for tee used as elbow, entering run

Question 13

Shell-and-tube exchanger tubing: Considering fouling services, what is the commonly recommended minimum outside diameter (OD) of tubes to facilitate cleaning and reduce blockage risk?

Accepted Answer

Introduction / Context: Tube outside diameter (OD) selection affects heat transfer area, pressure drop, mechanical strength, and cleanability. In fouling services (e.g., dirty cooling water, slurries), larger tube bores help slow fouling deposition and facilitate mechanical cleaning with brushes or scrapers. Given Data / Assumptions: Shell-and-tube exchanger handling fluids prone to fouling. Standard tube gauges (B.W.G. 14–16 are common). Mechanical cleaning options (rodding, brushing) are anticipated. Concept / Approach: Small tubes provide more area per shell diameter but clog more easily and are harder to clean. Industry practice often discourages very small OD tubes for fouling duties. A 3/4 inch OD (about 19 mm) tube is a widely recommended minimum for such services, balancing area density with maintenance practicality. Step-by-Step Solution: Identify fouling risk → prioritize cleanability.Select OD that allows passage of cleaning tools and reduces blockage frequency.Adopt 19 mm OD as a practical minimum for fouling fluids in many standards and vendor catalogs. Verification / Alternative check: Maintenance guidelines from exchanger suppliers and plant standards frequently call for 19 mm or larger OD in dirty services, with 25 mm (1″) common where space permits. Why Other Options Are Wrong: 16 mm: more prone to plugging and difficult to clean. 28 mm or 38 mm: feasible but not a “minimum”; larger ODs increase bundle size and cost. Common Pitfalls: Optimizing only for area density and ignoring maintainability; underestimating fouling factors and cleaning intervals; selecting thin gauges that deform under rodding. Final Answer: 19 mm

Question 14

Tray hydrodynamics: In actual operation of a distillation column, why is vapor flow often not uniformly distributed among bubble caps on a tray?

Accepted Answer

Introduction / Context: Even on well-designed bubble-cap or valve trays, actual vapor flow rarely splits evenly among all active devices. Understanding the causes of maldistribution helps troubleshoot capacity, efficiency, and weeping/entrainment issues. Given Data / Assumptions: Crossflow tray with an inlet downcomer and an outlet weir. Liquid flows across the tray from inlet to outlet. Vapor rises through caps from the plenum beneath the deck. Concept / Approach: As liquid traverses the tray, a hydraulic gradient (liquid height gradient) develops, typically deeper near the outlet weir. This gradient changes the local hydrostatic head above caps, altering the pressure drop that vapor must overcome. Caps under shallower liquid (upstream) pass more vapor; those under deeper liquid (downstream) pass less, causing maldistribution. Step-by-Step Solution: Recognize crossflow → liquid depth increases toward the weir.Hydrostatic head sets vapor bubbling resistance at each cap location.Greater head downstream → lower local vapor rate; lesser head upstream → higher local vapor rate. Verification / Alternative check: Tray rating tools model vapor–liquid hydraulics showing non-uniform froth height. Field gamma scans often reveal higher activity at the inlet side for this reason. Why Other Options Are Wrong: Low skirt clearance, low static submergence, or small downcomer seal are individual settings but not the primary driver of cap-to-cap maldistribution; the liquid gradient is. Common Pitfalls: Assuming perfect distribution; ignoring revamps that alter weir loading; overlooking tray leveling which can exacerbate gradients. Final Answer: Due to liquid height gradient across the tray

Question 15

Valve materials and size limits: Brass valves are typically manufactured and applied up to what nominal pipe size range (inches) in industrial service?

Accepted Answer

Introduction / Context: Valve body material selection balances corrosion resistance, mechanical strength, cost, and manufacturability. Brass is widely used for small utility and instrumentation services, but its application is usually limited to smaller nominal sizes. Given Data / Assumptions: General service valves (e.g., globe, gate, ball, needle) in non-severe conditions. Standard pressure classes; not specialized high-temperature or corrosive alloys. Focus on typical procurement practice rather than exceptional custom builds. Concept / Approach: Brass valves are favored in small sizes due to ease of casting and machining, sufficient strength for low-to-moderate pressure, and low cost. As sizes increase, weight, cost, and pressure class limitations make steel, bronze, or stainless bodies more appropriate. Hence, brass valves are commonly found up to about 2 inches nominal pipe size. Step-by-Step Solution: Establish service: utility/instrument air, water, nitrogen, non-aggressive fluids.Link size to material: brass practical and economical at small diameters.Conclude typical limit: up to and including 2 inches. Verification / Alternative check: Supplier catalogs list extensive brass offerings through 1/8″ to 2″, with larger sizes sparse or unavailable compared to bronze/steel lines. Why Other Options Are Wrong: ≤ 1 inch: too restrictive; many brass valves exist up to 2 inches. > 2.5 or > 3.5 inches: uncommon in brass for industrial duty due to strength and cost considerations. Common Pitfalls: Assuming brass suitability for high-temperature or high-pressure services; overlooking dezincification in aggressive waters where bronze or stainless may be preferred. Final Answer: ≤ 2 inches

Question 16

Vacuum evaporators and utility selection: Low-pressure steam (saturation temperature 105–120°C) is commonly used with vacuum levels of about how many millimetres of mercury (referenced to a 760 mm Hg barometer)?

Accepted Answer

Introduction / Context: Evaporators often operate under vacuum to reduce boiling temperature, protect heat-sensitive products, and improve energy efficiency. Selecting a suitable vacuum level aligns product properties, available steam temperature, and overall heat transfer coefficients. Given Data / Assumptions: Low-pressure steam with saturation temperature about 105–120°C is available. Vacuum level is referenced to 760 mm Hg (i.e., barometric). Goal is to obtain boiling at substantially reduced temperatures. Concept / Approach: A vacuum noted as 650 mm Hg on a 760 mm Hg barometer indicates the absolute pressure is near 110 mm Hg. At such low absolute pressures, water and many aqueous solutions boil at significantly lower temperatures, enabling use of low-pressure steam with adequate temperature driving force across the heating surface. Step-by-Step Solution: Define vacuum: vacuum value = 760 − P_abs (in mm Hg).For 650 mm Hg vacuum → P_abs ≈ 110 mm Hg → low boiling temperature.Match with steam at 105–120°C to provide a reasonable delta-T for heat transfer. Verification / Alternative check: Typical single- or multi-effect vacuum evaporators report vacuums of 600–700 mm Hg for aqueous solutions when using low-pressure steam utilities. Why Other Options Are Wrong: 250 or 450 mm Hg: higher absolute pressure, less temperature drop, less typical for low-pressure steam utilities. 750 mm Hg: extremely deep vacuum near barometric limit, uncommon for routine industrial operation. Common Pitfalls: Confusing absolute pressure with “vacuum” reading; failing to account for boiling point elevation in non-ideal solutions; ignoring non-condensable gas purging needs. Final Answer: 650 mm Hg

Question 17

Routing high-pressure fluids in shell-and-tube exchangers: For mechanical integrity and economy, where should the higher-pressure stream be placed?

Accepted Answer

Introduction / Context: Mechanical design strongly influences exchanger cost. The placement of the higher-pressure fluid determines required wall thicknesses, gasket stresses, and fabrication complexity. Choosing the tube side for high pressure is a common and economical practice. Given Data / Assumptions: Conventional fixed or floating head shell-and-tube exchanger. One stream has significantly higher design pressure. Materials and fouling tendencies do not mandate the opposite routing. Concept / Approach: The tube side consists of many small-diameter tubes with relatively thick walls that can withstand high internal pressures with modest metal thickness. By contrast, designing a large-diameter shell for high pressure demands very thick walls, heavier flanges, and more expensive fabrication. Therefore, routing the high-pressure stream through the tubes reduces cost and weight. Step-by-Step Solution: Evaluate pressure: identify the higher-pressure stream.Assess geometry: small tube diameters handle pressure efficiently.Select routing: place high-pressure stream in tubes unless other constraints (e.g., fouling, viscosity) dominate. Verification / Alternative check: Mechanical design equations show required thickness increases roughly with diameter for a given pressure; hence smaller tubes are favorable. Why Other Options Are Wrong: Shell side for pressure drop or heat transfer: routing is not primarily decided by pressure drop or U-values. Counter-current/co-current rule: flow arrangement is a thermal consideration, not a pressure containment rule. Common Pitfalls: Ignoring fouling/viscosity that might force the viscous fluid to the shell side; forgetting tube-side velocity limits for erosion or vibration. Final Answer: Tube side, to avoid heavy high-pressure shell construction

Question 18

Log-mean temperature difference (LMTD) correction: In which heat exchangers is the LMTD correction factor FT applied to account for departure from true counter-current or co-current flow?

Accepted Answer

Introduction / Context: The LMTD method assumes pure counter-current or co-current flow. Real exchangers often deviate due to multipass arrangements or crossflow geometries. The correction factor FT adjusts the ideal LMTD to reflect these departures for more accurate area sizing. Given Data / Assumptions: Shell-and-tube multipass, crossflow, and other non-ideal flow arrangements are common. Terminal temperature differences are known. FT ≤ 1 and depends on configuration and temperature effectiveness ratios. Concept / Approach: When flows are not purely counter-current or co-current (e.g., 1–2 shell-and-tube, 2–4 passes, crossflow with baffles), temperature profiles are less efficient, reducing the effective LMTD. FT is obtained from charts or equations based on the temperature ratio and capacity rate ratio. Step-by-Step Solution: Compute ideal LMTD from terminal temperatures.Identify exchanger configuration and obtain FT from charts.Calculate corrected driving force: LMTD_corrected = FT * LMTD_ideal. Verification / Alternative check: For a true 1–1 counter-current exchanger, FT = 1; for multipass/crossflow, FT typically ranges from about 0.7 to 0.95 depending on approach temperatures. Why Other Options Are Wrong: 1–1 co-current or double-pipe only: FT is generally 1 (or not needed) for ideal cases; real need is in multipass/crossflow. All liquid/liquid: phase has nothing to do with needing FT; geometry/arrangement does. Common Pitfalls: Applying FT outside its valid range; using FT too low (e.g., < 0.75) without reconsidering configuration; ignoring temperature cross limitations. Final Answer: All multipass configurations (and crossflow patterns)

Question 19

Atmospheric storage tank design: For tanks of diameter ≤ 15 m, what is the typical minimum shell plate thickness specified for elastic stability?

Accepted Answer

Introduction / Context: Atmospheric storage tanks must resist wind loads, handling damage, local buckling, and fabrication tolerances. Even when internal pressure is negligible, a minimum shell thickness is imposed for elastic stability and practical fabrication. Given Data / Assumptions: Cylindrical vertical tank with diameter ≤ 15 m. Design pressure near atmospheric; hydrostatic head due to stored liquid applies. Common industry practice and standards guide minimum gage selection. Concept / Approach: Thin shells are susceptible to elastic instability (local buckling or wrinkling) during construction, lifting, or service. A nominal minimum thickness (often around 5 mm) provides stiffness, allows welding without burn-through, and tolerates corrosion allowances and out-of-roundness within reasonable limits. Step-by-Step Solution: Identify governing load: not internal pressure but handling/wind and hydrostatic head.Apply practical fabrication constraints: plate availability, welding process, corrosion allowance.Adopt minimum shell thickness of about 5 mm for tanks ≤ 15 m diameter as a baseline. Verification / Alternative check: Industry guides and many company standards specify a lower bound of about 5 mm (exclusive of additional corrosion allowance) for small-to-medium atmospheric tanks. Why Other Options Are Wrong: 2 mm: too thin for handling/welding and prone to buckling. 10 or 15 mm: possible in special cases but not the minimum; adds unnecessary cost for small tanks. Common Pitfalls: Ignoring local shell buckling near nozzles; neglecting corrosion allowance; underestimating wind loads and required stiffening rings. Final Answer: 5 mm

Question 20

In shell-and-tube heat exchangers, what guideline is commonly used for minimum tube-to-tube clearance on the tubesheet (i.e., the free metal ligament between adjacent tube holes)?

Accepted Answer

Introduction / Context: In mechanical design of shell-and-tube heat exchangers, the spacing of tube holes in the tubesheet must ensure structural integrity during drilling, rolling/welding, and service. Designers refer to the minimum free metal ligament (tube-to-tube clearance on the tubesheet) to avoid cracking between holes, maintain gasket seating, and allow manufacturability. Given Data / Assumptions: Conventional shell-and-tube exchanger with drilled tubesheet. Pitch patterns may be square or triangular, but the question asks a general rule-of-thumb for minimum ligament. Objective: pick a conservative, widely used guideline. Concept / Approach: The free metal ligament equals the tube pitch minus tube outer diameter at the tubesheet. If the ligament is too small, machining weakens the sheet and may cause leakage or cracking. A common shop practice is to keep the ligament no less than one-fourth of tube OD or 3/16 inch, whichever is larger, as a practical lower bound for strength and manufacturing tolerance. Step-by-Step Solution: Recognize that “clearance” here refers to the free metal ligament on the tubesheet.Compare industry guidelines for minimum ligament vs. the options given.Select the option stating one-fourth of OD or 3/16 inch minimum, which matches common practice. Verification / Alternative check: Fabricator standards and exchanger handbooks frequently recommend minimum ligaments around 0.25*OD with an absolute floor near 3/16 inch for common tube sizes to ensure drillability and seating during expansion/welding. Why Other Options Are Wrong: More than one full OD or exactly one OD: unrealistic and uneconomical; would lead to very large pitch. “Always greater for triangular pitch”: triangular pitch typically has closer spacing than square for a given pitch; blanket statements are misleading. “No minimum clearance”: unsafe; ignores machining and strength constraints. Common Pitfalls: Confusing tube-to-tube clearance in bundle layout with shell-side clearance; ignoring corrosion allowance and drilling tolerances that further justify a conservative ligament. Final Answer: Not less than one-fourth of the tube outer diameter or 3/16 inch, whichever is larger

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